g = gpgrad(net, x, t)
g = gpgrad(net, x, t) takes a Gaussian Process data structure net together
with a matrix x of input vectors and a matrix t of target
vectors, and evaluates the error gradient g. Each row
of x corresponds to one input vector and each row of t
corresponds to one target vector.
gp, gpcovar, gpfwd, gperrCopyright (c) Ian T Nabney (1996-9)